The present invention relates to secondary, rechargeable batteries, particularly such batteries which are constructed of layered, polymeric composition electrode and electrolyte elements laminated with electrically-conductive collector members, typically metallic foils. More particularly, the invention relates to such batteries comprising reticulate collector foils and provides a means for reducing the internal electrical resistance factor of such batteries which may, in part, be attributable to insulating metallic oxides formed on the surface of such collector foils, as well as the insulating effect of electrolyte solution wetting the electrode/collector foil interface.
Typical laminated polymeric composition battery structures with which the present invention is useful are described, for example, in U.S. Pat. Nos. 5,460,904 and 5,478,668. Such a battery comprises respective positive and negative polymeric matrix electrode composition layers of lithium intercalation compound and carbon which are laminated together and to metal foil current collector elements that provide the primary terminals for electrical connections.
As is known in the industry, the individual electrical resistance of each member of a battery structure contributes to an overall internal battery resistance which represents a nonproductive load and energy drain in any utilization circuit, particularly one which includes an external low impedance device. The power dissipated in overcoming such internal resistance not only detracts directly from the efficiency of a battery, it may further generate within the battery a level of heat which has a deleterious effect on not only the operation of the battery, but also on the integrity of the battery members, viz., the electrodes and electrolyte. Such effects are particularly felt by polymeric members of the noted laminated lithium ion rechargeable batteries.
A significant source of electrical resistance has been observed in the oxide which readily forms on the surface of the current collector foils, particularly aluminum, preferably employed with the polymer matrix lithium intercalation compound and carbon electrode compositions of battery cells such as described in the above-noted patents. Also contributing to the resistance in these cells has been the introduction of activating electrolyte solution which results in a swelling and expansion of the electrode members and intrusion of the solution between the electrode and collector surfaces, thereby interfering with the firm physical contact which ensures good electrical conductivity through these members.
The present invention provides an effective means of substantially eliminating the formation of insulating metal oxides on the collector elements, as well as of maintaining the integrity of a strong physical, electrically-conductive bond between the electrode and collector members, and thereby dramatically reducing the internal resistance of the Li-ion intercalation battery cells which are gaining favor in the industry.